There are many methods and products available for changing the natural color of hair. Semi-permanent or temporary dyeing methods, or direct dyeing methods, temporarily change the color of hair. These methods can change the color of the hair to varying degrees and the color change may withstand several rounds of shampooing. Many consumers seek more permanent results, and therefore default to oxidative dye products that contain hydrogen peroxide or other oxidants. In order to provide the consumer with the shade, longevity, and the intensity of color desired, an oxidative coloring process is utilized. Permanent hair dyeing formulations typically include primary intermediates (also known as oxidative hair dye precursors or developers) and couplers (also known as color modifiers or secondary intermediates). These dye precursors are sufficiently small, polar and soluble to diffuse into the hair shaft where, once activated by an oxidizing agent under basic conditions, such as hydrogen peroxide, the primary intermediates react with other dye precursors, e.g., couplers, to form larger colored chromophores in the hair shaft. The chromophores formed in the hair shaft do not readily diffuse from the hair during subsequent washing.
The oxidative coloring of hair can require long processing times. For instance, oxidative coloring processes involve premixing a coloring base and a developer. This mixture is then applied to the hair and must remain on the hair for a long period of time (an extended “processing” time) to potentiate the desired color change. Direct dyes, however, do not require admixing and activation by oxidizing agents and do not require long processing times.
Many attempts have been made by the hair color industry to enhance the washfastness (tenacity) of direct dyes by either forming a covalent bond between chromophore and proteins inside hair or increasing the number of binding sites, typically cationic centers, on the chromophore. However, each attempt has its drawbacks. The approach through covalent bonding does not differentiate proteins in hair from skin. The approach through multiple binding sites on the dyes (i.e. multiple positive charges to interact with negative sites on hair, either by bonding several monocationic dyes together or by installing multiple cationic centers on a single chromophore) runs into the obstacles of uneven color due to uneven damage (negative charges) along the length of the hair fibers and reduced dye penetration into hair fibers because the dyes are typically at least twice as large as common oxidative dye precursors. An increase in the number of binding sites minimizes bleeding and color loss caused by rinsing by providing stronger hair-chromophore interactions. However, the same strong binding force to the cuticle also prevents the chromophores from penetrating deep into the cortex of hair, because it is difficult for dyes with multiple positive charges to diffuse through negatively charged networks of keratin proteins. Additionally, since polycationic dyes remain bound to the hair surface rather than penetrating into the fiber, it is difficult to produce dark shades, due to limited binding sites on the surface of hair.
Furthermore, the dying processes using direct dyes typically involve application of creams containing the direct dyes to the hair. These creams must be massaged with the hands or combed-into the hair to provide uniform coverage. This process can be messy and time consuming. Improving the washfastness (tenacity) of direct dye products and simplifying the coloring process is desirable to both consumers and to professional hair stylist.